1. Field of the Invention
The present invention relates to an FM receiver for receiving text-based multiplex broadcasts.
2. Description of the Related Art
In FM broadcasting, for example, in Japan, what is referred to as "the FM text-based broadcasting", in which data, such as character information, is multiplexed and transmitted with a normal sound broadcast program, has been implemented.
The above FM text-based broadcasting is referred to as "the data radio channel (DARC) system", which has been developed by NHK Science & Technical Research Laboratories. In this DARC system, digital-data, such as character-information, multiplexing standards are as follows.
Sub-carrier frequency: 76 kHz PA0 Transmission rate: 16 kbits/second PA0 Modulation method: level controlled minimum shift keying (LMSK) PA0 Error correcting system: (272,190) product code by compacted differential-set cyclic codes PA0 0: undefined PA0 1: general information for sequential-receiving processing PA0 2: general information for record-receiving processing PA0 4: traffic information PA0 0: table of contents (main menu) PA0 1-253: general text-based programs (news, weather forecasts, etc.) PA0 254: major complementary programs (program information or sub-program) PA0 255: emergency information
An LMSK signal is frequency-multiplexed with an audio signal (monoral signal or a stereo composite signal) of a main sound broadcasting program, and the frequency-multiplexed signal is then transmitted.
The program services, such as character information, are differentiated by Levels 1, 2 and 3, all of the levels displaying (presenting) character information in the form of dots. Level 1 is a service targeted for FM receivers which are capable of displaying 15.5 characters.times.2.5 lines, including a header, and the information given by this level includes characters. Level 2 is a service aimed for FM receivers which can display 15.5 characters.times.8.5 lines, and the information presented by this level includes characters and graphics. Further, level 3 is a service provided for FM receivers which are capable of displaying detailed maps by means of CD-ROMs, i.e., level 3 is a traffic information service for a navigation system.
The program services at level 1 may include news, weather forecasts, traffic information, entertainment, major complementary programs, etc. Among the above services, entertainment services provide fortune-telling, listener's messages, quizzes, town information, and so on.
Further, the major complementary programs provide complementary information for main programs. For example, if a main sound broadcasting program is a music program, the complementary program provides the music titles, the performers, and the telephone and FAX numbers, which enable the listener to make a request. The major complementary programs will be hereinafter referred to as "the program information" or "the sub-program".
In addition to the above-described program services, "emergency information" may be provided as required in the case of emergency.